This invention relates to cathode-ray tubes having apertured shadow masks therein, and particularly to a shadow mask construction which reduces misregister between electron beams and phosphor elements of the tube screen caused by expansion of the shadow mask and which also facilitates construction of a flat faceplate cathode-ray tube.
In a shadow mask type cathode-ray tube for producing a color image, a plurality of convergent electron beams are projected through a multi-apertured color selection shadow mask to a mosaic screen. The beam paths through the mask are such that each beam impinges upon and excites only one kind of color-emitting phosphor on the screen. Generally, the shadow mask is attached to a rigid frame, which in turn, is suspended within the picture tube envelope.
When a color cathode-ray tube is operated, the electrons that strike the shadow mask cause it to heat up. Since the edges of the shadow mask are attached to a somewhat heavy frame which serves as a heat sink, a temperature differential developes between the center and peripheral portions of the mask. Because of the temperature differentials, the mask center, the mask edge and the frame expand at different rates. This difference in expansion rates causes a doming of certain portions of the mask toward the screen. In the center of the screen, doming causes little effect on the register between the electron beams and phosphor elements because the straight line projection of the beams to the elements remains unchanged with changes in mask to screen spacing. Since the edges of the mask are fixed to a peripheral frame, there is no doming at the mask edges. Therefore, maximum misregister caused by doming occurs approximately halfway between the mask center and mask edge. Misregister is defined as being the amount of electron beam is off-center from its respective phosphor element. Because of this doming, the electron beams passing through the mask misregister with the phosphor elements of the screen. The misregister effect of doming peaks after 3 to 5 minutes of tube operation but continues to have a diminishing effect on tube performance for an additional 10 to 15 minutes. Once the tube has reached steady state temperatures, general electron beam misregister caused by expansion of the mask is compensated by temperature sensitive frame supports which move the mask-frame assembly toward the screen. Such temperature compensating support is disclosed in U.S. Pat. No. 3,803,436 issued to me on Apr. 9, 1974.
Another problem somewhat related to doming is blister warpage. Blistering occurs during operation of the tube and is caused by a video pattern, such as a sustained white spot in the TV image, that developes localized heating of a part of the mask.